"The Mysterious Cough, Caught on Film"

From The New York Times

The Mysterious Cough, Caught on Film

By DENISE GRADY

Published: October 27, 2008 

In Roald Dahl’s novel “The B.F.G.,” the title character, a big friendly giant, captures dreams in glass jars. At Pennsylvania State University, a professor of engineering has captured something less whimsical but no less ephemeral — a cough — on film. 
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The image, published online Oct. 9 by The New England Journal of Medicine, was created by schlieren photography, which “takes an invisible phenomenon and turns it into a visible picture,” said the engineering professor, Gary Settles, who is the director of the university’s gas dynamics laboratory. 

Schlieren is German for “streaks”; in this case it refers to regions of different densities in a gas or a liquid, which can be photographed as shadows using a special technique.

“In my lab we use this technique a lot,” Dr. Settles said. “Often it’s used for other things, like in supersonic wind tunnels, to show shock waves around high-speed aircraft.”

The process involves a small, bright light source, precisely placed lenses, a curved mirror, a razor blade that blocks part of the light beam and other tools that make it possible to see and photograph disturbances in the air. In the world of gas dynamics, a cough is merely “a turbulent jet of air with density changes.” Though coughs spread tuberculosis, SARS, influenza and other diseases, surprisingly little is known about them. “We don’t have a good understanding of the air flow,” Dr. Settles said.

To map a cough, he teamed up with Dr. Julian Tang, a virus expert from Singapore. A healthy student provided the cough. The expelled air, traveling at 18 miles per hour, mixed with cooler surrounding air and produced “temperature differences that bend light rays by different amounts,” Dr. Settles said. 

He went on: “The next thing is, you get a couple of people in front of the mirror talking, or one coughs on another, and you see how the air flow moves, how people infect one another. Or you look at how coughing can spread airborne infection in a hospital. This is really a suggestion for how we might study all that. The techniques used in wind tunnels can be used to study human diseases.” 

Other schlieren images show the churning air and shock waves that emanate from a pistol’s firing; an Airedale sniffing a small flower; and the unseen, shimmering world around a candle burning in a breeze. 

The final photograph, in a full-scale mock-up of an aircraft cabin, captures in microseconds the flash of an explosion under a mannequin in an airplane seat and the propagation of shock waves into the cabin. The blast was a re-creation of a terrorist’s attempt in 1994 to bring down a Philippine Airlines flight with a nitroglycerin bomb. The plane did not crash, but the explosion did kill the passenger seated over the bomb. The simulation used a less intense explosion than the actual bombing.

“The simulation helps to understand how the energy of an onboard blast reverberates around the cabin,” Dr. Settles said, “and it is also useful to check the results of computer blast simulations.”